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Tess Vigeland: This week, we kick off our annual series on philanthropy, looking at trends affecting the nonprofit world. One of the huge trends overall this year is social media. Facebook and Twitter have already revolutionized the way we communicate with each other. Turns out, they're also changing how we give to charities. Last year, social media accounted for more than $2 billion in donations.

The Salvation Army has been ringing bells like that for more than a century to get people to drop spare change into its little red kettles. But about 10 years ago, donations to those kettles started to fall. With the proliferation of credit and debit cards, people didn't always have change on hand.

George Hood oversees development for the Salvation Army.

George Hood: My cynical side about five years ago said, there's going to be a day when these bell ringers and the red kettles are going to start to disappear.

So the Salvation Army began recruiting virtual bell-ringers through Facebook and Twitter.

Hood: You get a virtual kettle that has your name on it, and then you e-mail blast all of your friends and ask them to donate to the Salvation Army through your red kettle.

The effort generates nearly $3 million a year. And it's helped change the way the Salvation Army raises money. Hood says the group's traditional donor base is aging out. So the Salvation Army decided to phase out radio advertising and use that money to bolster its presence online.

Hood: We know that our future is dependent on connecting with emerging generations. And we know that we can't communicate with these emerging generations through newspapers and radios much longer. They're going to the Internet to get all of their information.

Social media has also been essential to the success of small, start-up charities. Take ShelterBox USA. The group is part of a British charity that ships boxes with tents, water purifiers and other basics to places hit by disasters.

Veronica Brandon Miller had just started as executive director at the U.S. office in January when Haiti was hit by a massive earthquake.

Veronica Brandon Miller: There were two people in the office, me and the database manager.

Moments after the quake, they jumped online, sending out requests for help through Facebook and Twitter. Prominent bloggers soon picked up on their efforts.

Miller: And 43 minutes later -- it was very interesting -- CNN called my cell phone to see what we were doing. And I'm like, "We are already on our way."

ShelterBox was eventually able to provide supplies for a quarter of a million Haitians. In the months after the earthquake, the group went from less than a million dollars in donations to more than $10 million.

Miller: We had absolutely no money for getting our word out there. So social media really saved us.

And it might just be helping to save that age-old practice of ringing bells for the Salvation Army.

Sound of bell ringing

The group says since it started using social media, more donations are piling up in those real red kettles outside stores. That bell, by the way, is the Salvation Army's iPhone app.